Preparation of BiFeO3 thin films by pulsed laser deposition method
ZHANG Guan-jun(张冠军), CHENG Jin-rong(程晋荣), CHEN Rui(陈 蕊),
YU Sheng-wen(俞圣雯), MENG Zhong-yan(孟中岩)
School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China
Received 10 April 2006; accepted 25 April 2006
Abstract: BiFeO3 (BFO) thin films were prepared on Pt(111)/TiO2/SiO2/Si(100) substrates by the pulsed-laser deposition (PLD) technique at a low temperature of 450℃. The XRD results indicate that the BFO thin films are of perovskite structure with the presence of small amount of second phases. The oxygen pressures have great effect on the crystalline structures and dielectric properties of BFO thin films. The dielectric constant of the BFO thin films decreases with increasing oxygen pressures, achieving 186, 171 and 160 at the frequency of 104 Hz for the oxygen pressures of 0.666, 1.333 and 13.332 Pa, respectively. The BFO thin films prepared at the oxygen pressure of 0.666 Pa reveal a saturated hysteresis loop with the remanent polarization of 7.5 ?C/cm2 and the coercive field of 176 kV/cm.
Key words: BiFeO3 thin films; Pt(111)/TiO2/SiO2/Si(100) substrates; PLD; structural and dielectric properties
1 Introduction
Multiferroic materials that exhibit long rang electric as well as magnetic ordering were studied for a new type of memory application using a combination of ferroelectric and ferromagnetic properties[1]. Of particular interest is BiFeO3 (BFO), which exhibits the coexistence of ferroelectric and antiferromagnetic (G-typed) orders up to quite high temperatures[2, 3]. In bulk single crystals, BFO is reported to exhibit the ferroelectric Curie temperature TC of 1 103 K, and the magnetic transition temperature TN of 643 K[4, 5]. BiFeO3 is of a rhombohedrally distorted perovskite, which belongs to the space group R3c[6-8]. The rhombohedral unit cell parameters are of ar =3.96 ? and α =89.45?. The unique structure-related properties of BiFeO3 potentially offer a whole range of applications, including the emerging field of spintronics, data-storage media and multiple-state memories [9].
Although BFO is currently emerging as a prime candidate for room-temperature ferroelectromagnets, there remain several issues to be solved before it is practically implemented to the fabrication of multiferroic devices operating at room temperature. The issues include electrical leakage[10, 11], small spontaneous polarization, inhomogeneous magnetic spin structure and ferroelectric reliability.
In this work, BiFeO3 thin films were investigated in terms of their structure and dielectric properties. Specially, the effect of oxygen pressures on the phase evolution and dielectric properties of BFO thin films were examined at room temperature.
2 Experimental
BiFeO3 thin films with thickness of around 230 nm were prepared on Pt(111)/TiO2/SiO2/Si(100) substrates by pulsed laser deposition at the deposition temperature of 450 ℃. A KrF excimer laser (TuiLaser/Thin Film Star-100) with a 248 nm wavelength was used to ablate the target materials with a repetition rate of 5 Hz. The laser energy density was 2.5 J/cm2. BFO thin films were deposited using sintered ceramics of BiFeO3 which were batched with 10%(in mole fraction) excess Bi2O3 to compensate for the loss of bismuth during growth. The oxygen was utilized as the deposition ambient. The film thickness was measured by using an Alpha-step 500 profiler. X-ray diffraction (XRD) meter (Rigaku-D/MAX-2000) was employed to analyze the crystalline structures. Pt electrodes were sputtered on the surface of BFO through a shadow mask with a diameter of 0.2 mm. Dielectric constant and loss tangent were measured using a precision impedance analyzer (HP 4192A). The leakage current density of BFO thin films were measured by HP 4140B pA meter/DC voltage source. The ferroelectric hysteresis loop was examined by using the ferroelectric measurement system (Radient, RT6000 HVS).
3 Results and discussion
Fig.1(a) shows the X-ray diffraction patterns of BFO thin films grow on Pt(111)/TiO2/SiO2/Si(100) substrates under the various oxygen pressures. The XRD pattern of target is in Fig.1 (b). The major diffraction peaks coming from both BFO thin films and the target can be indexed as perovskite BFO. There are no significant differences from XRD patterns under different oxygen pressures. However, the intensity of diffraction peaks increases with oxygen pressures, indicating that the oxygen pressure promotes the growth of crystalline BFO thin films at the low deposition temperature. The presence of small amount of Bi2Fe4O9 impurity phase was detected from the BFO thin films. Moreover, the Bi2O2.75 phase was detected in the ceramic target. It is concluded that the BFO thin films can be crystallized impurity phases in BFO thin films might
Fig.1 XRD patterns of (a) BiFeO3 thin films prepared at 450 ℃ under oxygen pressures of 0.666, 1.333 and 13.332 Pa (b) BiFeO3 target
ascribe to the into perovskite at the temperature as low as 450 ℃. The presence of the second phases in the BFO target. The vitalization of bismuth might occur in the sintering of ceramics leading to the elements out of stoichiometric ratio[12].
Fig.2 shows the dielectric constant κ and tan δ of BFO thin films as a function of frequency at room temperature. It is found that the values of κ and tan δ slightly change with frequency in the range of 104-106 Hz. The dielectric constant decreases with increasing oxygen pressures. The value of tan δ is below of 0.05 at the frequency of less than 106 Hz. The BFO thin films deposited at the oxygen ambient of 0.666 Pa reveal a relatively higher dielectric constant of 186 compared with those of 171 and 160 for BFO thin films prepared at 1.333 and 13.332 Pa respectively. It is indicated that the BFO thin films possess good dielectric properties at room temperature.
Fig.2 Dielectric constant and tan δ of BiFeO3 thin films deposited at oxygen ambient of 0.666, 1.333 and 13.332 Pa
Fig.3 shows the leakage current density J of BFO thin films under different oxygen pressures as a function of applied field E. It can be seen that BFO thin films deposited at 0.666 and 1.333 Pa have lower leakage current density than that of the film deposited at 13.332 Pa. For the lower oxygen ambient, the leakage current density of BFO thin films is around 10-6 A/cm2 under the field of 100 kV/cm. The lower deposition oxygen pressure might improve the roughness of the film surface contributing to the lower leakage current density. Unfortunately, the J-E curve did not reveal good ohmic characteristics, indicating that the film-electrode interface was not in the prefect contact. Moreover, the value of J increased greatly with the applied fields, reflecting that the presence of defects in the BFO thin films could not be neglected.
Fig.3 Leakage current density of BiFeO3 thin films deposited under oxygen ambient of 0.666, 1.333 and 13.332 Pa
Fig.4 represents the ferroelectric hysteresis loops of BFO thin films deposited at 0.666 Pa. The saturated hysteresis loops evidenced the ferroelectricity of our BFO thin films. The remanent polarization and coercive field of BFO thin films were of 7.5 ?C/cm2 and 176 kV/cm respectively under the applied field of 400 kV/cm.
Fig.4 Ferroelectric hysteresis loops of BFO thin films deposited under oxygen ambient of 0.666 Pa
4 Conclusions
BiFeO3 thin films were deposited on Pt(111)/TiO2/SiO2/Si(100) substrate by pulsed laser deposition. BiFeO3 thin films could be crystallized on the perovskite structure at 450 ℃. The impurity phase existing in BFO thin films ascribed to the presence of the second phases in the BFO target. Lower deposition pressures contributed to the better dielectric and ferroelectric properties of BFO thin films. The results indicate that BFO thin films have reasonable dielectric and ferroelectric properties. The further improvements of the film-electrode interface need to be made to reduce the leakage current density of BFO thin films.
Acknowledgements
We are pleased to acknowledge support from National Nature Science Foundation of China under Grant Nos. 50472098, 50332030 and 50302006, Shanghai Rising Star Program under Grant No. 04qmx1440, SHMEC Grants No. 04AB18 and the Key Subject Construction Project (Material Science) of Shanghai Educational Committee.
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(Edited by HE Xue-feng)
Foundation item: Project (50472098) supported by the National Natural Science Foundation of China
Corresponding author: CHENG Jin-rong; Tel: +86-21-56332704; E-mail: jrcheng@staff.shu.edu.cn
